Field
The presently disclosed subject matter relates to a two-dimensional optical deflector. The optical deflector can be applied as an optical scanner to a laser pico projector, a laser radar, a bar code reader, an area sensor, an adaptive drive beam (ADB) type head lamp, a head-up display unit, and other optical apparatuses, to generate scanning light.
Description of the Related Art
Generally, in an optical scanner or the like, an optical deflector is constructed by a micro electromechanical system (MEMS) device manufactured by using semiconductor manufacturing processes and micro machine technology.
A prior art two-dimensional optical deflector as a MEMS device is constructed by a mirror, a pair of torsion bars coupled to the mirror along an axis (X-axis), an inner frame (movable frame) surrounding the mirror and the torsion bars, inner piezoelectric actuators coupled between the torsion bars and supported by the inner frame via inner coupling portions, serving as cantilevers for rocking the mirror with respect to the X-axis of the mirror, an outer frame (fixed frame) surrounding the inner frame, and outer piezoelectric actuators coupled between the inner frame and the outer frame, serving as cantilevers for rocking the mirror along another axis (Y-axis) of the mirror (see: FIG. 19 of JP2008-040240A). The inner piezoelectric actuators are driven by a relatively high frequency such as 25 kHz for a horizontal scanning, while the outer piezoelectric actuators are driven by a relatively low frequency such as 60 Hz for a vertical scanning.
The above-described prior art two-dimensional optical deflector includes a single silicon-on-insulator (SOI) structure which is constructed by a monocrystalline silicon support layer (“Handle” layer), an intermediate (buried) silicon dioxide layer (“Box” layer) and a monocrystalline silicon active layer (“Device” layer) (see: FIGS. 25, 26, 27 and 28 of JP2008-040240A). In this case, the mirror, the torsion bars, the inner frame, the inner piezoelectric actuators and the outer piezoelectric actuators have the same thickness (height) determined by the active layer of the SOI structure. Also, a lead titanate zirconate (PZT) layer is deposited at a high temperature of 500° C. to 600° C. on the active layer of the SOI structure, and the PZT layer is patterned by a photolithography and dry etching process. On the other hand, a part of the active layer of the SOI structure is etched and exposed, and a metal is deposited as a mirror on the exposed part of the active layer of the SOI structure.
In the above-described prior art two-dimensional optical deflector, however, since the thickness of the outer piezoelectric actuators cannot be decreased while maintaining the thickness of the mirror, the torsion bars and the inner piezoelectric actuators, the frequency of the outer piezoelectric actuators would not be decreased. On the contrary, since the thickness of the inner piezoelectric actuators cannot be increased while maintaining the thickness of the outer piezoelectric actuators, the frequency of the inner piezoelectric actuators would not be increased.
Also, deposition of PZT and etching the deposited PZT may damage the surface of the active layer, so that large residual stresses would be generated in the active layer and its surface would be roughened. Therefore, the quality of the mirror would deteriorate due to the damaged surface of the active layer.